Figure 11.17
Phase Changes
Phase Changes
Figure 11.18
Phase Changes
•
•
•
•
•
•
Energy Changes Accompanying
Phase Changes
Sublimation: ∆Hsub > 0 (endothermic).
Vaporization: ∆Hvap > 0 (endothermic).
Melting or Fusion: ∆Hfus > 0 (endothermic).
Deposition: ∆Hdep < 0 (exothermic).
Condensation: ∆Hcon < 0 (exothermic).
Freezing: ∆Hfre < 0 (exothermic).
Prentice Hall © 2003
Chapter 11
Phase Changes
Energy Changes Accompanying
Phase Changes
• All phase changes are possible under the right conditions.
• The sequence
heat solid → melt → heat liquid → boil → heat gas
is endothermic.
• The sequence
cool gas → condense → cool liquid → freeze → cool solid
is exothermic.
Prentice Hall © 2003
Phase Changes
Energy Changes Accompanying
Phase Changes
• Generally heat of fusion (enthalpy of fusion) is less than
heat of vaporization:
– it takes more energy to completely separate molecules, than
partially separate them.
Chapter 11
Phase Changes
Heating Curves
• Plot of temperature change versus heat added is a heating
curve.
• During a phase change, adding heat causes no
temperature change.
– These points are used to calculate ∆H fus and ∆Hvap.
Prentice Hall © 2003
Chapter 11
Prentice Hall © 2003
Chapter 11
Vapor Pressure
Explaining Vapor Pressure on the
Molecular Level
Figure 11.22
Figure 11.19
Prentice Hall © 2003
Chapter 11
Prentice Hall © 2003
Vapor Pressure
Phase Changes
Critical Temperature and Pressure
• Gases liquefied by increasing pressure at some
temperature.
• Critical temperature: the minimum temperature for
liquefaction of a gas using pressure.
• Critical pressure: pressure required for liquefaction.
•
•
•
•
Prentice Hall © 2003
Chapter 11
Explaining Vapor Pressure on the
Molecular Level
Dynamic Equilibrium: the point when as many molecules
escape the surface as strike the surface.
Vapor pressure is the pressure exerted when the liquid
and vapor are in dynamic equilibrium.
Volatility, Vapor Pressure, and Temperature
If equilibrium is never established then the liquid
evaporates.
Volatile substances evaporate rapidly.
Prentice Hall © 2003
Vapor Pressure
•
•
•
•
Explaining Vapor Pressure on the
Molecular Level
Some of the molecules on the surface of a liquid have
enough energy to escape the attraction of the bulk liquid.
These molecules move into the gas phase.
As the number of molecules in the gas phase increases,
some of the gas phase molecules strike the surface and
return to the liquid.
After some time the pressure of the gas will be constant
at the vapor pressure.
Prentice Hall © 2003
Chapter 11
Chapter 11
Chapter 11
Vapor Pressure
Volatility, Vapor Pressure, and
Temperature
• The higher the temperature, the higher the average kinetic
energy, the faster the liquid evaporates.
Figure 11.23
Prentice Hall © 2003
Chapter 11
Vapor Pressure
Phase Diagrams
Vapor Pressure and Boiling Point
• Liquids boil when the external pressure equals the vapor
pressure.
• Temperature of boiling point increases as pressure
increases.
Prentice Hall © 2003
Chapter 11
Prentice Hall © 2003
Vapor Pressure
Vapor Pressure and Boiling Point
• Two ways to get a liquid to boil: increase temperature or
decrease pressure.
– Pressure cookers operate at high pressure. At high pressure the
boiling point of water is higher than at 1 atm. Therefore, there
is a higher temperature at which the food is cooked, reducing
the cooking time required.
• Normal boiling point is the boiling point at 760 mmHg (1
atm).
Prentice Hall © 2003
Chapter 11
Phase Diagrams
Prentice Hall © 2003
Chapter 11
The Phase Diagrams of H2O and CO2
Figure 11.26
Figure
11.27
Chapter 11